Microwave radios must occupy an allocated radio frequency (RF) spectrum and are required to do so using a high-stability reference oscillator. Additionally, in several terrestrial communication systems, user equipment or terminals must acquire and retain time synchronization and frequency synchronization with a central communication node. Thus, it is known that nodes or elements belonging to a radio communications network need to be synchronized with a common time reference. This is particularly important in respect of communications networks that serve mobile telephones, where one and the same time reference must be used between different base stations in order for a user to be able to move without hindrance between the areas covered by base stations during an ongoing communication.
Known solutions to this problem may be found in the use of autonomous clocks or in clocks that are locked to an available navigational system, such as the Global Positioning System (GPS). However, it is not always possible to use GPS, because it may not be possible to receive requisite signals, such as in the case of underground base station installations.
It is also known that oscillators may be controlled automatically with respect to frequency by automatically assigning to the oscillator a correction value in accordance with a given periodicity. Exemplary oscillators that may be controlled automatically are voltage controlled oscillators (VCOs) in which a voltage level determines the frequency of the oscillator. In the case of an oscillator of this kind, it is possible to calibrate and adjust the oscillator periodically and automatically, by controlling the voltage level in question. However, this requires a relevant correction value to be given, i.e., a voltage level that corresponds to the frequency to which the oscillator shall be set.
Oscillators such as VCOs may also be coupled in a phase-locked relationship with a reference signal. For example, when the VCO forms a portion of a PLL circuit, the oscillation frequency of oscillating signals generated by the VCO are locked to that of a reference signal to which the VCO is operably responsive. Many types of radio communication apparatuses utilize VCOs coupled in PLL circuits. For example, a radio base station operable in a cellular communication system is exemplary of a radio communication apparatus which utilizes a VCO coupled in a PLL circuit. Oscillating signals formed by the VCOs are used to form transmit signals which are transmitted by a transmitter apparatus. Additionally, oscillating signals generated by VCOs of receiver apparatuses, for instance, are used in the reception of radio signals. Acceptable frequency stabilities of the oscillating signals generated by the VCOs are required for proper operation of the radio communication apparatus. Acceptable frequency stability of oscillating signals generated by the VCO is required so that downlink signals generated by the radio base station are properly transmitted to a mobile terminal without interfering with other concurrently-transmitted downlink signals. Acceptable levels of frequency stability are similarly required to permit the radio base station to properly receive uplink signals transmitted by mobile terminals to the radio base station.
Thus, while the process of acquiring and retaining synchronization with a central communication node may impose strict requirements on a terminal's frequency standard or clock, a shortcoming of the use of high-precision frequency standards is the associated expense of such oscillators. Furthermore, the problems of transferring a synchronizing reference signal may be divided into two separate groups. First, it may be necessary for an absolute time to be known, i.e., the time of day, and second, a local oscillator may be required to oscillate at the same frequency as a specified reference frequency within a given error tolerance. Embodiments of the present subject matter relate to allowing oscillators active in a network to oscillate at a common frequency.
Network-generated signals, such as a pulse code modulated (PCM) clock signal or a GPS clock signal, may be provided to a communication network. Such signals exhibit good long-term, frequency stability characteristics but are susceptible to short-term, frequency instability. Oscillators, such as an oven-voltage controlled crystal oscillator (OVCXO) or oven controlled crystal oscillator (OCXO) may generate a reference signal which exhibits good short-term frequency stability characteristics but is susceptible to long-term frequency instability due to aging of the crystal oscillator.
Thus, for reasons of cost, autonomous clocks are normally comprised of quartz oscillators. These clocks, however, require periodic manual calibration in order to be able to generate a time reference signal within set requirements. Low cost oscillators such as quartz oscillators have numerous shortcomings including an oscillator's gradual change in frequency over long periods of time. This phenomenon is known as aging. The aging rate of a quartz oscillator is a fundamental limitation in its use in a terminal/central node communication system. Additionally, the performance of quartz oscillators is affected by temperature variation, the range of oscillator frequency as a function of the temperature of the oscillator; retrace, the difference in frequency measured immediately before turn-off and again after turn-on and stabilization; and warm-up, the time measured from initial application of power, required for a crystal oscillator to stabilize its frequency to within specified limits.
Thus, to ensure that the frequency stability standards required of operation of communications networks are met, some networks include reference oscillators which generate reference signals of comparable quality. An OVCXO or OCXO is exemplary of such an oscillator. OVCXO and OCXOs, as well as other oscillators, exhibit short-term frequency stability, but are susceptible to long-term frequency drift caused by aging of the oscillator. Conventionally, such oscillators must be calibrated regularly. Calibration is typically carried out utilizing a manual procedure. Such a procedure is costly, particularly when large numbers of radio base stations of a radio communication system must all be regularly calibrated.
A need has thus arisen for a system and method to provide a means for calibration of a terminal or RF frequency standard. A further need has arisen for a system and method by which the good long-term frequency stability characteristics of a standard provided by a network may be used to correct for the frequency offset of an oscillator positioned at a reference element or site to thereby reduce the need to manually calibrate the oscillator. More generally, a need exists by which to permit a device to be calibrated with a remote reference signal.
Accordingly, it is an object of the present subject matter to obviate many of the deficiencies in the prior art and to provide a novel method of calibrating a local radio reference clock for a radio operating in a radio network having a network reference clock. The method comprises the steps of determining at the radio an offset between the local clock and the network clock, placing the local clock in a calibration mode, and calibrating the local clock using a radio link to reduce the offset.
It is also an object of the present subject matter to provide a novel method of calibrating a local radio reference clock for a radio operating in a radio network having a network reference clock for a synchronous communication protocol. The method comprises calibrating the local radio clock with the network reference clock by communicating information to the radio over a radio communication channel.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.